The present invention relates to power amplification in electronic equipment, and more particularly to methods and apparatuses for reducing the peak-to-average-power ratio of a signal to be amplified.
Power amplification remains an issue of much research in electronics, and is especially important in telecommunications. Where telecommunications equipment is operated by a time-limited power supply, such as a battery (e.g., in a mobile phone or other User Equipment—“UE”), the efficiency of the power amplifier (PA) that amplifies the signal to be transmitted (e.g., uplink signals sent by UE to a base station—“BS”) largely determines the talk time of the equipment. The PA is by nature a non-linear component, as can be seen from the typical power transfer function (Pout vs. Pin) shown in FIG. 1. In order not to distort the signal, the signal amplitude excursions should remain in the linear region (e.g., the region to the left of the dotted line 101 in FIG. 1). Distortion of the signal gives rise to spectral (re-)growth outside the signal bandwidth. Signal power is thereby spread outside the intended bandwidth, which gives rise to leakage into one or more adjacent channels.
Signals that have little variation in amplitude (so-called “constant-envelope signals”) are therefore preferred because the operating point of the PA can be placed high in the linear region, where the efficiency of the PA is high. If the signal amplitude varies much, the operating point of the PA has to be moved downwards, so that strong signal excursions will still remain in the linear region. But, by backing-off the operating point of the PA, its efficiency is detrimentally lowered.
The ratio between the maximum signal excursion and the average excursion of a signal is expressed by the Peak-to-Average-Power Ratio (PAPR). The PAPR is therefore a measure of the extent to which peak values of a signal are larger than typical values. In mobile telephony, modulation formats with a low PAPR have been very popular. For example, the Global System for Mobile communication (GSM) uses Gaussian Minimum Shift Keying (GMSK) modulation, which results in a modulated signal having a PAPR of 0 dB because its amplitude remains constant (the information is represented only in the phase of the signal: Continuous Phase Modulation or CPM). However, in order to increase the data rate and obtain higher link capacities (in b/s/Hz), higher-order modulation (HOM) is unavoidable. This requires not only that the phase be modulated, but the amplitude as well, resulting in larger PAPR. For example, modulation schemes up to 64-QAM have been introduced in systems complying with the High Speed Downlink Packet Access (HSDPA) standards. Similarly, modulation schemes up to 16-QAM have are being investigated for use in High Speed Uplink Packet Access (HSUPA) systems.
Very high PAPR levels are found in multi-carrier technologies like Orthogonal Frequency Division Multiplexing (OFDM). These technologies have gained popularity in new systems under development like those compliant with the Worldwide Interoperability for Microwave Access (WiMAX) standards and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). In LTE, the standard working group has chosen a multi-carrier, OFDM scheme for use in the downlink transmissions (from BS to UE), but a single-carrier modulation scheme (QPSK, 16-QAM, and 64-QAM) for use in the uplink transmissions. The latter has a much better PAPR than OFDM. PA efficiency is crucial in the battery-powered UE.
Although single-carrier modulation schemes are associated with better (i.e., lower) PAPR values than do multi-carrier modulation schemes like OFDM, the need for efficiency can still present problems in single-carrier modulation-based equipment because higher-order modulation technologies generate signals having a higher PAPR than do constant-envelope modulation technologies like GMSK. In particular, for higher data rates, higher power levels are required in order to keep the energy per bit at a reasonable level (i.e., sufficient to cover the distance that the radio waves need to travel). A high PA efficiency is, therefore, mandatory not only to achieve efficient battery power usage, but also to keep the heat caused by power dissipation at reasonably low levels. Therefore, there is a need for methods and apparatuses that will reduce the PAPR of modulated signals in single-carrier communication systems.